The ever-increasing demand for optical bandwidth continues to drive the need for faster data transmission rates, which then requires improvements in system performance for parameters such as optical signal to noise ratio (OSNR). While some attempts in improving OSNR involve changes in the coding schemes utilized to perform the data transmission, other attempts are directed to improving the optical properties of the transmission channel itself. For example, amplification of the optical signal along a transmission span using the technique of distributed Raman amplification (DRA) is becoming prevalent in a variety of different optical communication systems.
While useful, DRA requires the use of relatively high power pump sources to initiate the generation of gain within the transmission medium (e.g., power levels higher than 500 mw are routinely required). The presence of high power light has safety implications, both for the personnel installing the DRA in the optical communication system as well as for the actual waveguide (e.g. fiber) transporting the signal. For example, before provisioning a DRA, it is important to know if there are any “breaks” or other types of flaws along the associated fiber span where the amplification is to take place. This is especially a concern in the central office (CO) environment, where high Raman pump powers could emit unsafe levels of optical power and expose various pieces of equipment to damage (as well as personnel to injury at the CO location). These concerns, among others, have prompted the search for a way to deploy DRAs in a manner that is as safe as possible.
Moreover, the ability to determine the fiber characteristics, such as attenuation or Raman gain coefficient, are important as part of the analysis involved in optimizing the parameters required to achieve the best performance from a DRA system that is being installed along a span of optical fiber. Adding intelligence into optical components, such as embedding an optical time domain reflectometry (OTDR) measurement capability, is important in determining key system factors associated with achieving as much signal amplification as possible.
In conventional situations, OTDR measurements are made upon the installation of a new DRA system (before sending “live” traffic over the fiber) so that the personnel performing the installation will have an understanding of the quality of the span. The data collected by the OTDR measurement may then be used, for example, to ensure that the fiber is of the high quality necessary to support the utilization of high power Raman pumps. Various other characteristics of the fiber can be measured and used to assess the amount of amplification that may be achieved.
In operation of an exemplary OTDR measurement system, a pulse of light is injected into a section of optical fiber and the reflections coming back towards the pulse source are captured and measured over a period of time. The reflections can be used to determine what losses exist along the fiber, such as those associated with Fresnel reflections (at interruptions such as connectors, splices, or the like) or Rayleigh backscatter (which is the reflection derived from the nature of the fiber/waveguide structure itself).
Inasmuch as it is relatively expensive and time-consuming to deploy an engineer to perform the OTDR measurements at the location of a new installation, there has been an on-going effort to develop various types of remote testing. While advances have been made that utilize a Raman pump source itself to create the pulses used for OTDR measurements, this type of testing can only be performed when the system is otherwise not in service (i.e., when the Raman pump sources are not being used as create gain in propagating transmission signals). While this condition is met for the situation of testing new installations, there remains a need to perform on-going OTDR measurements for maintenance reasons, such as monitoring the health of the fiber and determining if there is a degradation in performance over the operational lifetime of the system. For this collection of long-term data, it is not feasible to periodically take a portion of a system out of service, perform an OTDR measurement, and then bring the system back on line, as would be required if the conventional method of using Raman pumps was employed.